Photolithographic techniques have long been employed in the microelectronics and information industries to transfer information from a modulated light beam to a recording medium in the form of a surface relief pattern. The recording medium contains a photoresist, which is a material that changes its solubility in certain solvents after exposure to light. Thus, after exposure, the more soluble portions of the resist can be removed by contact with a developer solvent.
As these industries have become more sophisticated, the need for recording information at ever-increasing density has become apparent. Since electron beams can be more highly focussed than light beams, they allow for increased density of information and their use is supplanting light beams for recording. However, since materials which are sensitive to light beams are not necessarily sensitive to electron beams, new electron beam sensitive materials are being sought.
Thompson and Bowden, J. Electrochem. Soc., Vol. 120, December 1973, pp. 1722-1726, have disclosed that copolymers of olefins and SO.sub.2 in film form are effective positive-working electron beam resists. These copolymers are spun or cast from solution onto a substrate, exposed to a modulated, information carrying electron beam and developed by treatment with a suitable solvent. As is known, a careful choice of developer solvent must be made for each polysulfone to maximize the resolution of the developed film. In the case of very fast working solvents solvent-nonsolvent mixtures can be employed to reduce undue erosion of the unexposed portions of the resist.
Electron beam recording has been found to be very useful in the preparation of masters for a grooved record which carries audio, video and color information in the form of a surface relief pattern in the bottom of the small groove. Such recording has been disclosed in U.S. Pat. No. 3,842,217 to Clemens. The requirements for an acceptable positive electron beam resist for this application are stringent. The resist must be highly sensitive, that is undergo a large change in solubility when struck by electrons. In addition, since resist films for this application are employed in comparatively thick layers that are not developed through to the substrate, as contrasted with thinner resist films employed as masks which are developed through to the substrate for subsequent etching of the substrate, the present resist films must also be capable of high resolution whereby, after development, the surface relief pattern must have straight-walled troughs or developed regions. The sharper the geometry of the relief pattern, the higher the quality of the recording. As a further requirement, it is desired that the unexposed portions of the resist be highly insoluble to the developer solvent. When information corresponding to both audio and video information, which varies greatly in frequency, is recorded in the resist, the height of the unexposed portion of the resist should remain constant after development to increase the resolution of the recorded information.
Thus a sensitive electron beam resist which can be applied in comparatively thick films, and a process for developing it to form straight-walled surface relief patterns of high resolution, without dissolving the unexposed resist, has been sought.